1. Field of the Invention
The present invention relates to an ultrasonic motor capable of driving a driven object with a multi-degree of freedom.
2. Description of the Related Art
An ultrasonic motor is known which applies a voltage to an ultrasonic vibrator to thereby friction drive a driven object kept in contact with the vibrator. In this case, if the driven object is made spherical to extract output therefrom, the ultrasonic motor can be used as a driving source of a multi-degree of freedom. Therefore, various applications of the ultrasonic motor, such as incorporation of the motor in a robot arm or a multi-joint arm, are expected.
As the ultrasonic motor capable of driving a driven object with a multi-degree of freedom, Jpn. Pat. Appln. KOKAI Publication No. 2005-143176, for example, has proposed a rotary driving device equipped with a driven object, rotary member support means, alternation power supply means, a vibrator, vibrator support means, rotation friction reduction means and pressing force adjusting means. The driven object has a substantially spherical surface. The rotary member support means supports the driven object so that the object can rotate about at least two axes. The alternation power supply means supplies electric energy to the vibrator. The vibrator converts, into mechanical energy, the electric energy supplied from the alternation power supply means, thereby generating a three-dimensional vibration. The vibrator support means supports the vibrator so that the vibrator will contact the substantially spherical surface of the driven object at a preset angle. The rotation friction reduction means reduces the friction that occurs when the driven object is rotated. The pressing force adjusting means adjusts, to a preset value, a pressing force with which the driven object and the vibrator contact each other.
In the rotary driving device disclosed in the above publication, the central portion of the upper surface (i.e., the output surface) of the vibrator is in point contact with the spherical surface of the driven object from the relationship between the output surface of the vibrator and the spherical surface of the object. Using the elliptic motion of the vibrator as a driving source, the driven object is friction driven about the contact point.
The vibrator will abrade at the contact point at which the vibrator and the spherical surface of the driven object directly contact each other. Further, the contact point will shift in accordance with the abrasion. Thus, the operation of the driving device is unstable.
To suppress the abrasion of the vibrator, the above publication has proposed to provide a sliding member on the upper portion of the vibrator. However, since the sliding member also abrades in accordance with the rotation of the vibrator, it does not fundamentally solve the abrasion problem.